Surface treatment of solar cells

ABSTRACT

Methods of fabricating emitter regions of solar cells using surface treatments, and the resulting solar cells, are described herein. In an example, a method of fabricating a solar cell includes treating a surface of a silicon substrate to form a lyophilic area between two lyophobic areas and depositing a liquid phase material containing a silicon material in the lyophilic area to form an emitter region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.16/344,176, filed on Apr. 23, 2019, which is a 371 of internationalPCT/IB2017/001424, filed on Oct. 17, 2017, which claims benefit of U.S.Provisional Application No. 62/417,223, filed on Nov. 3, 2016.

TECHNICAL FIELD

Embodiments of the present disclosure are in the field of renewableenergy and, in particular, forming emitter regions using surfacetreatment.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a back-contact solar cellhaving emitter regions formed above a back surface of a substrate.

FIG. 2 illustrates a cross-sectional view of N-type and P-type emitterregions of a back-contact solar cell.

FIG. 3 illustrates a flowchart of a method of fabricating a solar cellusing surface modification of a wafer substrate to define an emitterregion structure.

FIGS. 4A-4B illustrate operations of a method of fabricating a solarcell using surface modification of a wafer substrate to define anemitter region structure.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics can be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” lyophobic area does not necessarily imply that this lyophobicarea is the first lyophobic area in a sequence; instead the term “first”is used to differentiate this lyophobic area from another lyophobic area(e.g., a “second” lyophobic area).

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

“Inhibit”—As used herein, inhibit is used to describe a reducing orminimizing effect. When a component or feature is described asinhibiting an action, motion, or condition it can completely prevent theresult or outcome or future state completely. Additionally, “inhibit”can also refer to a reduction or lessening of the outcome, performance,and/or effect which might otherwise occur. Accordingly, when acomponent, element, or feature is referred to as inhibiting a result orstate, it need not completely prevent or eliminate the result or state.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper,” “lower,” “above,”and “below” refer to directions in the drawings to which reference ismade. Terms such as “front,” “back,” “rear,” “side,” “outboard,” and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

To provide context, back-contact solar cells include n-type areas for anegative contact and P-type areas for a positive contact, and the areasare accessible at the rear side of the solar cell, i.e., the side thatis not directly exposed to incoming sunlight. The n-type areas andP-type area can be interdigitated, and may be referred to as emitterregions or “fingers.” Finger structure morphology can influence solarcell function, and accordingly, controllable formation of the fingerstructure is generally desirable. Formation of the finger structures,however, can be a cumbersome process. Despite challenges in forming thefinger structures, however, back-contact solar cells are nevertheless anattractive product, as they offer advantages, for example related toperformance and reliability. Accordingly, reductions in the complexityof forming back-contact solar cells, and methods of defining orcontrolling emitter region structures, is generally desirable.

In one embodiment, a method of fabricating a solar cell includestreating a surface of a silicon substrate to form a lyophilic areabetween two lyophobic areas. The method can include depositing a liquidphase material, e.g., a silane-type polymer, on the surface in thelyophilic area to form an emitter region between the two lyophobicareas. The emitter region can be a P-type doped region or an N-typedoped region.

FIG. 1 illustrates a cross-sectional view of a back-contact solar cellhaving emitter regions formed above a back surface of a substrate. Asolar cell includes a silicon substrate 100 having a light-receivingsurface 102. A passivating dielectric layer 108 can be disposed on thelight-receiving surface of the silicon substrate 100. An optionalintermediate material layer (or layers) 110 can be disposed on thepassivating dielectric layer 108. An anti-reflective coating (ARC) layer119 can be disposed on the optional intermediate material layer (orlayers) 110, as shown, or can be disposed on the passivating dielectriclayer 108.

On the back surface of the substrate 100, alternating P-type 120 andN-type 122 emitter regions are formed. In one such embodiment, trenchesare disposed between the alternating P-type 120 and N-type 122 emitterregions. More particularly, in an embodiment, first polycrystallinesilicon emitter regions 122 are formed on a first portion of a thindielectric layer 124 and are doped with an N-type impurity. Secondpolycrystalline silicon emitter regions 120 are formed on a secondportion of the thin dielectric layer 124 and are doped with a P-typeimpurity. In an embodiment the tunnel dielectric 124 is a silicon oxidelayer having a thickness of approximately 2 nanometers or less.

Conductive contact structures 128/130 are fabricated by first depositingand patterning an insulating layer 126 to have openings and then formingone or more conductive layers in the openings. The conductive contactstructures 128/130 include metal and can be formed by a deposition,lithographic, and etch approach or, alternatively, a printing or platingprocess or, alternatively, a foil or wire adhesion process.

Referring to FIG. 2, a cross-sectional view of N-type and P-type emitterregions of a back-contact solar cell are shown. The N-type 202 andP-type 204 fingers, i.e., the emitter regions, can be disposed on a thintunnel oxide 206 on a wafer 200. The trench region 208 can be doped andextend between the emitter region fingers. For example, the trenchregion 208 can be doped N-type. An oxide layer 210 can cover the entirestructure, which can be primarily a PSG-layer, i.e., a mixture of SiO2and P2O5.

Referring to FIG. 3, a flowchart of a method of fabricating a solar cellusing surface modification of a wafer substrate to define or control anemitter region structure is shown. FIGS. 4A-4B illustrate operations ofa method of fabricating a solar cell, which can be exemplary operationsof the method shown in FIG. 3. Thus, the following description refers toFIG. 3 in combination with the operations shown in FIGS. 4A-4B.

In an embodiment, a thin tunnel oxide on a wafer substrate is covered bya thin layer of silicon. The thin layer of silicon can have an exposedsurface, e.g., a top surface. The thin layer of silicon may be referredto as a silicon substrate. Patterns of lyophobic areas and/or lyophilicareas can be formed on the silicon substrate prior to deposition of aliquid phase material that forms the emitter region fingers.

By way of introduction, the terms lyophobic and lyophilic are usedthroughout this description to refer to surface properties of anunderlying region of the silicon substrate. An area of the substratesurface is lyophilic when it has wetting surface properties for aparticular material, which as described below, can be a liquid phasematerial used to form emitter regions, e.g., an N-type or P-type emitterregion. An area of the substrate surface is lyophobic when it hasnon-wetting surface properties for the particular liquid phase material.Thus, the terms lyophilic area and lyophobic area make sense in thecontext of the material that is being deposited onto the areas. This canbe compared and contrasted with the terms “hydrophilic” and“hydrophobic,” which presume that the deposited material is water. Thatis, a hydrophilic area can have wetting surface properties for water,but can nonetheless be lyophobic to a different material of interest.For example, an oxide layer can be hydrophilic in that it has wettingsurface properties for water, but has non-wetting surface properties fora liquid phase material containing a silicon material. Therefore, itwill be understood that the formation of lyophobic and lyophilic areasas described below can be achieved using different processes that modifythe silicon substrate surface to have wetting or non-wetting surfaceproperties for a given material, which can be different than silicon inan embodiment.

At operation 301, the exposed surface of the silicon substrate istreated to form a lyophilic area between two lyophobic areas. Forexample, the exposed surface of the silicon substrate can originally belyophilic, and a first lyophobic area and a second lyophobic area can beformed by modifying the exposed silicon surface in a manner that causesthose areas to become lyophobic. The first lyophobic area can belaterally separated from the second lyophobic area. The untreated areasbetween the first and second lyophobic area can be the lyophilic area.

In an alternative embodiment, the substrate material, e.g., the siliconsubstrate, can originally be lyophobic, and the lyophilic area can beformed by modifying the substrate material. More particularly, theexposed silicon surface can be modified in a manner that causes thetreated area to become lyophilic. The newly formed lyophilic area canlaterally separate the first lyophobic area from the second lyophobicarea.

Referring to FIG. 4A, in an embodiment, treating the surface 405 of thesilicon substrate 404 (which may be a silicon layer 404 on an oxidelayer 402 on an underlying substrate or wafer 400, as depicted, or maybe a surface of a bulk monocrystalline silicon substrate), includesforming an oxide layer 406 on the silicon substrate to form the two ormore lyophobic areas 408. Forming the oxide layer 406 can includehydrogenating all or part of the silicon substrate surface, and thenlocally oxidizing regions of the hydrogenated surface to form thelyophobic area.

Hydrogenating the silicon substrate surface can include terminating theexposed lyophilic surface 410 of the silicon substrate with hydrogen.For example, the surface can be exposed to a hydrogen-terminating agent,such as hydrofluoric acid, which terminates the silicon surface of thesubstrate with hydrogen. Alternatively, the silicon surface can beexposed to a hydrogen-plasma to generate hydrogen-terminated surfaces.

Forming the oxide layer 406 can also include exposing thehydrogen-terminated surface to an oxygen-containing atmosphere, andlocally stimulating oxidation by energy supply such as from UV-radiationor laser irradiation. The oxide layer can be lyophobic to a depositionmaterial of interest, e.g., a liquid phase material that forms theemitter finger region, as discussed below. Thus, forming the oxide layercan modify the originally lyophilic substrate surface such that thetreated areas become lyophobic to form the two lyophobic areas.

The hydrogen-terminated surface of the silicon substrate can be oxidizedby locally exposing the surface of the silicon substrate to energy in anoxygen-containing atmosphere. The energy can include energy fromultraviolet (UV) radiation, laser, or plasma.

In an embodiment, the treated areas that are transformed into lyophobicareas are illuminated with a laser in air or in oxygen. Laserillumination can be applied in patterns, e.g., by scanning a laser beamover areas where the lyophilic condition of the treated areas is to beturned into a lyophobic condition. The local illumination can form athin oxide layer to provide the lyophobic properties to the treatedareas.

In an embodiment, the treated areas are illuminated with UV radiation inan air or oxygen-containing atmosphere. UV illumination can be appliedin a desired pattern using suitable optics. Alternatively, a shadow maskcan be used to control exposure of the silicon surface to the UVradiation. The shadow mask can be laid directly on the silicon substratesurface, and thus, the UV radiation can be limited to the areas wherethe lyophilic condition of the treated areas is to be turned into alyophobic condition. The local illumination can form a thin oxide layerto provide the lyophobic properties to the treated areas.

In an embodiment, the oxygen-containing atmosphere is a general ozoneenvironment. A mask can be placed onto the silicon substrate surfaceprior to exposing the wafer to the general ozone environment. The wafercan then be exposed to the general ozone environment. The mask canshield underlying areas of the silicon substrate surface from thegeneral ozone environment, and the unmasked areas can be oxidized by thegeneral ozone environment to form a thin oxide layer to provide thelyophobic properties to the treated areas.

In an embodiment, the treated areas are exposed to an oxygen plasma.Atmospheric plasma treatments can be less capital expenditure-intensive,e.g., as compared to vacuum processing. Atmospheric plasma treatmentscan change wetting behavior of surfaces and can be controlled to achievea desired lyophilic/lyophobic condition of the treated areas. A desiredpattern can be formed using a shadow mask. The shadow mask can be laiddirectly on the silicon substrate surface, and thus, the plasmatreatment can be limited to the areas where the lyophilic condition ofthe treated areas is to be turned into a lyophobic condition.

The treatment options discussed above, i.e., UV radiation exposure,laser exposure, or plasma exposure (in which the plasma exposure caninclude, e.g., an oxygen plasma, an N2O plasma, or an inert gas plasma),can accelerate oxide formation on silicon. More particularly, thetreatment options can generate lyophobic silicon surfaces by modifyinghydrogen-terminated surfaces of a lyophilic silicon surface. Theexposure can be minimal. That is, exposure can be limited to an amountof exposure required to achieve a desired change to surface properties,regardless of a resulting oxide thickness. The exposure of thehydrogen-terminated surface to oxidizing conditions can generate a thinlayer of oxide. For example, one or more of the lyophobic area caninclude an oxide layer having one or more monolayers of oxide togenerate a lyophobic surface. In an embodiment, the oxide layer hasfewer than ten monolayers of oxide. Accordingly, the oxide layer canturn the lyophilic condition of the treated areas into a lyophobiccondition.

To clarify again, although an embodiment is presented above in which anoriginally lyophilic surface is modified to form two lyophobic surfaceareas, in another embodiment, an originally lyophobic surface ismodified to form a lyophilic surface area between two originallylyophobic areas. In any case, after treating the substrate surface, thesurface have a pattern including a lyophilic area between two lyophobicareas. Furthermore, although this description has been limited to aportion of the substrate surface having a single lyophilic area betweentwo lyophobic areas, it will be appreciated that the method can bescaled to form a larger pattern having several lyophilic areas betweenrespective groupings of two lyophobic areas. For example, the modifiedsurface of the silicon substrate can include a sequential arrangement oflyophilic areas separated from each other by a single lyophobic area,and lyophobic areas separated from each other by a single lyophilicarea.

The different pattern areas of the surface, i.e., the lyophilic areabetween the lyophobic areas, can have different surface conditions. Thatis, the lyophilic areas can be lyophilic to a material of interest, andthe lyophobic areas can be lyophobic to the material of interest. Thisdifference in surface conditions can define stopping areas where amaterial deposited in a liquid phase does not enter. Thus, patternedareas can control and define a structure of a deposited material asdescribed below.

At operation 303, a liquid phase material can be deposited on thesurface in the lyophilic area to form an emitter region between the twolyophobic areas. The liquid phase material used to form an emitterregion can be a liquid phase material containing silicon. For example,the liquid phase material may include a silane-type polymer dispensed ina solvent. Alternatively, the liquid phase material may include siliconnanoparticles dispensed in a solvent. Accordingly, the liquid phasematerial may include any liquid phase material including a siliconmaterial that can be deposited as a pre-cursor for forming a siliconlayer. In an embodiment, the deposited liquid phase material can behigh-temperature annealed, e.g., at a temperature below 1400 degreesCelsius, to form a silicon finger morphology.

In an embodiment, the liquid phase material may not include the actualsilicon that forms an emitter region finger, however, the liquid phasematerial can nonetheless include dopants. More particularly, the emitterregion finger can be formed by dispensing the liquid phase material ontothe silicon substrate, and dopants contained within the liquid phasematerial may diffuse into the silicon substrate to form the emitterregion fingers in the silicon substrate.

Referring to FIG. 4B, an amount of the liquid phase material 412 can bedeposited in the lyophilic regions 410 between the first lyophobic areaand the second lyophobic area 408.

As material is deposited into the lyophilic area between the lyophobicareas, the edges of the material can eventually contact the lyophobicareas, causing a shape of the deposited material edges 414 to change.That is, the deposited material, e.g., silicon dispensed in solvent, canspread over the entire lyophilic area in some cases. When an amount ofdeposited material is greater than an amount necessary to fill thelyophilic area, the deposited material can form a steep contact angle420 where it borders against the lyophobic areas. The edges of thematerial deposited into the lyophilic area can be thin and knife-like asthe material spreads within the lyophilic area, but upon contacting thelyophobic area, the lateral spreading of the material can stop and thematerial edges can thicken. The liquid phase can be held back by thelyophobic properties of the oxide of the lyophobic areas. Surfacetension can laterally constrain the deposited material such that thedeposited film has a substantially constant thickness between lateraledges, and each edge is steep. For example, each edge may have athickness that drops to zero within a lateral distance that is of asimilar or smaller magnitude than the film thickness. As onenon-limiting example, each edge may have a thickness that drops to zerowithin a lateral distance no greater than 10 times, e.g., no greaterthan 1.25 times, the thickness. The lateral distance can be less thanthe thickness. In an embodiment, each edge 414 includes a convexity 416that bulges outward from the substrate surface. The deposited materialcan form an emitter region 418 having a thick edge with an abrupt changeof the layer thickness to zero at the edge, as described above.

In a similar manner, using different wetting properties of the depositedsilicon fingers and the trench regions in between, it is possible toapply a self-aligned dielectric to a surface of the emitter regionfingers. In the envisaged scenario, the dielectric is deposited via aliquid or viscous phase. Deposition can include dipping, ink jetting, orscreen printing, to name a few options. The deposited material will, ifthe wetting properties of the involved surfaces are suitably designed,wet only the intended finger regions and not cover the trench.

The above description provides a pathway for using a promising strategyof a simple production sequence for back-contact solar cells. Localdeposition of a liquid phase material containing a silicon material,such as printing a silane-type polymer, can provide a promising strategyfor forming emitter regions on a silicon substrate of the solar cells.The described method can form the emitter region structures to includepredetermined structural contours, e.g., edges having an abrupt changeof thickness down to zero. Furthermore, the emitter region structuredefined through the use of patterned areas having different wettingsurface properties can provide a pathway for low-cost and simpleproduction of effective back-contact solar cells.

Thus, methods of fabricating solar cells using surface treatment of awafer substrate to control an emitter region structure, and theresulting solar cells, have been disclosed.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of the present disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of the present application (or an applicationclaiming priority thereto) to any such combination of features. Inparticular, with reference to the appended claims, features fromdependent claims may be combined with those of the independent claimsand features from respective independent claims may be combined in anyappropriate manner and not merely in the specific combinationsenumerated in the appended claims.

What is claimed is:
 1. A solar cell fabricated according to a method,the method comprising: treating a surface of a silicon substrate to forma lyophilic area between a first lyophobic area and a second lyophobicarea; and subsequent to treating the surface of the silicon substrate toform the lyophilic area, depositing a liquid phase material containing asilicon material on the surface in the lyophilic area to form an emitterregion between the first lyophobic area and the second lyophobic area,wherein the liquid phase material containing the silicon material doesnot form on the first and second lyophobic areas during the depositing.2. The solar cell of claim 1, wherein treating the surface to form thefirst lyophobic area and the second lyophobic area includes forming anoxide layer.
 3. The solar cell of claim 2, wherein treating the surfaceto form the first lyophobic area and the second lyophobic area includes:hydrogenating the surface; and exposing the hydrogenated surface to anoxygen-containing atmosphere.
 4. The solar cell of claim 2, whereinforming the oxide layer comprises using oxidation by energy suppliedfrom UV-radiation or laser irradiation.
 5. The solar cell of claim 1,wherein the liquid phase material includes one or more of a silanepolymer or silicon nano-particles.
 6. The solar cell of claim 1, whereinthe surface of the silicon substrate is a surface of a silicon layer ona silicon oxide layer on a silicon wafer.
 7. The solar cell of claim 1,wherein the surface of the silicon substrate is a surface of amonocrystalline silicon substrate.
 8. A solar cell fabricated accordingto a method, the method comprising: treating a lyophilic surface of asilicon substrate to provide a lyophilic area between a first lyophobicarea and a second lyophobic area; and subsequent to treating the surfaceof the silicon substrate to form the lyophilic area, depositing a liquidphase material containing a silicon material on the surface in thelyophilic area to form an emitter region between the first lyophobicarea and the second lyophobic area, wherein the liquid phase materialcontaining the silicon material does not form on the first and secondlyophobic areas during the depositing.
 9. The solar cell of claim 8,wherein treating the lyophilic surface to form the first lyophobic areaand the second lyophobic area includes forming an oxide layer.
 10. Thesolar cell of claim 9, wherein treating the lyophilic surface to formthe first lyophobic area and the second lyophobic area includes:hydrogenating the lyophilic surface; and exposing the hydrogenatedsurface to an oxygen-containing atmosphere.
 11. The solar cell of claim9, wherein forming the oxide layer comprises using oxidation by energysupplied from UV-radiation or laser irradiation.
 12. The solar cell ofclaim 8, wherein the liquid phase material includes one or more of asilane polymer or silicon nano-particles.
 13. The solar cell of claim 8,wherein the lyophilic surface of the silicon substrate is a surface of asilicon layer on a silicon oxide layer on a silicon wafer.
 14. The solarcell of claim 8, wherein the lyophilic surface of the silicon substrateis a surface of a monocrystalline silicon substrate.
 15. A solar cellfabricated according to a method, the method comprising: treating alyophobic surface of a silicon substrate to provide a lyophilic areabetween a first lyophobic area and a second lyophobic area; anddepositing a liquid phase material containing a silicon material on thesurface in the lyophilic area to form an emitter region between thefirst lyophobic area and the second lyophobic area.
 16. The solar cellof claim 15, wherein the lyophobic surface of the silicon substrate is asurface of a silicon layer on a silicon oxide layer on a silicon wafer.17. The solar cell of claim 15, wherein the lyophobic surface of thesilicon substrate is a surface of a bulk monocrystalline siliconsubstrate.